Method of operating an injection molding machine with continuously operating plasticizing unit

ABSTRACT

In a method of operating an injection molding machine of a type having an injection plunger for pushing melt from an injection space anteriorly of an injection plunger into an injection mold, a plasticizing screw of an extruder is continuously operated to continuously feed melt through a melt channel into the injection space. A passage through the melt channel is closed to allow backup of melt backwards into the extruder, and the injection plunger is moved forward to inject a shot amount from the injection space into the injection mold.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a division of prior filed copending U.S. applicationSer. No. 10/272,122, filed Oct. 15, 2002, which in turn is acontinuation of prior filed copending PCT International application no.PCT/EP01/04250, filed Apr. 12, 2001, which designated the United Statesand on which priority is claimed under 35 U.S.C. §120, the disclosure ofwhich is hereby incorporated by reference, and which claims the priorityof German Patent Application, Serial No. 100 18 542.8, filed Apr. 14,2000, pursuant to 35 U.S.C. 119(a)-(d), the contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates, in general, to an injection unit forinjection molding machines with continuously operating plasticizingunit.

Injection molding machines are known in which a reciprocating screwequipped with a back flow valve implements the plastification of theplastic granulate and generates the injection pressure. Other approachesinvolve an operational division between a screw plasticizing unit, onthe one hand, and a plunger-type injection unit, on the other hand.Constructions of this type have the advantage that a screw plasticizingunit can operate at optimum plasticizing capacities. Moreover, the useof several plunger-type injection units, fed from a screw plasticizingunit, permits the screw plasticizing unit to be continuously operatedbecause melt can be fed alternatingly to the various, normally two,plunger-type injection units. A continuous operation of the plasticizingunit is advantageous because not only is the melt more homogenous, butthe plasticizing capacity can be increased since shutdown periods areeliminated or a smaller screw can be utilized while maintaining a samecapability. Furthermore, wear is significantly reduced, in particularwhen large screws are involved, because there is no need for a constantre-starting (absence of adhesive wear during start-up). Also the motorand the transmission are subject to less stress in view of the evenload.

A screw plasticizing unit in combination with plunger-type injectionunits can also be used as compounder for mixing varied components. Asthe plasticizing screw is not moved back and forth, solid and liquidadditives can always be fed at the same location relative to the screw.The same is true, when producing foamed products, because foaming agentcan also be added always at the same spot. The continuous operationprevents an undesired pressure drop of the foaming agent, as hasbeen-experienced with injection molding machines with reciprocatingscrews during shutdown period.

An injection molding machine with a separate plasticizing unit, on theone hand, and several plunger-type injection devices, on the other hand,suffers however the drawback that the plurality of existing plunger-typeinjection devices requires significant space for installation. Moreover,long melt channels that need to be heated, and respective pressurelosses have to be accepted. Further, the alternating feeding of theplunger-type injection units require reverse valves. Typically, also thepurging quality is poor because the cylinder spaces of the plunger-typeinjection units cannot be filled and emptied according to the principalof a continuous melt stream flowing in only one direction (first-in,first-out).

German Pat. No. DE 195 05 984 A1 discloses an injection molding machinewith separate and continuously operating plasticizing unit and twoplunger-type injection devices, but with unidirectional melt flow(first-in, first-out). This publication illustrates an injection moldingmachine in which a screw plasticizing unit, comprised of twoplasticizing screws running in a same direction, is connected downstreamto two plunger-type injection devices which, controlled by reversevalves, can be fed alternatingly with melt. A unidirectional melt flowis realized in this injection molding machine by supplying eachplunger-type injection device through conduction of melt viatelescopically movable melt channels into the respective plunger bottomof the plunger-type injection devices. This significantly increases,however, the length of the melt channel.

International patent publication WO08/06321 discloses an injectionmolding machine for manufacturing injection-molded or extruded formedparts of mixed materials. Examples of mixed materials include herebyplastics with additional components such as fibers, particles, bindersor other additives. The plastic granulate is melted in a twin ormultiple screw compounder. The additional components are also added tothe compounder and mixed with the plastic melt as homogeneously aspossible. A melt channel extends from the forward end of the compounderand connects into the rear region of an injection space in which aninjection plunger is guided for reciprocating movement. Melt flowsthrough a back flow valve, arranged at the forward end of the injectionplunger, and fills the injection space in front of the injection plungerto thereby push the injection plunger backwards. When enough melt fillsthe injection space, the compounder is stopped and the supply of melt isinterrupted. Melt, stored in the injection space, is injected into theinjection mold as the injection plunger is moved forward throughapplication of a pressure medium. After filling the mold cavity, anafterpressure plunger, disposed downstream of the injection space andengaging the melt exit channel, which extends to the mold cavity, blocksthe melt exit channel, connected to the injection space, and,optionally, to generate an afterpressure. This conventional injectionmolding machine includes an unidirectional melt flow (first-in,first-out) and avoids long melt paths, when several plunger-typeinjection devices are involved (like, for example, in German Pat. No. DE195 05 984 A1). It is, however, disadvantageous that the compounder hasto be stopped during the injection phase and afterpressure phase. Thisadversely affects the melt quality because only a continuous operationof the compounder (or extruder) provides for a homogeneous meltthroughout. Moreover, the provision of back flow valves isdisadvantageous when processing melts filled with long fibers or naturalfibers because a flow through narrow and wound openings in the back flowvalve entails a risk of damage of the fibers.

It would be desirable and advantageous to provide an improved aninjection unit for injection molding machines to obviate prior artshortcomings and to allow a permanent continuous operation of theplasticizing unit regardless of the cycle time in combination with anintermittent operation of the injection molding process.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, an injection unit foran injection molding machine, includes a plasticizing unit in the formof a continuously operated extruder having at least one plasticizingscrew, a plunger-type injection device including an injection nozzle forinteraction with an injection mold, and an injection plunger demarcatinganteriorly thereof an injection space which is connected to a forwardend of the extruder by a melt channel; and a shutoff valve disposed inthe melt channel, wherein the extruder is so configured that a backuplength is able to expand backwards into the extruder, when the shutoffvalve is closed.

The present invention resolves prior art problems by combining aninjection plunger with an extruder having one or more plasticizingscrews, whereby the extruder is so configured that melt can backupfurther to the rear of the extruder, when a shutoff valve in the meltchannel is closed, so that the extruder itself offers a sufficientlylarge buffer area for “temporarily storing” melt during the injectionphase and afterpressure phase. This is especially true during theproduction of thinwalled formed parts, when the cooling period isrelatively short, so that the extruder can be operated continuously andmelt can be buffered in the free space between the screws.

According to another feature of the present invention, the extruder maybe a single screw extruder including a barrel, which is defined by acylinder diameter and accommodates the plasticizing screw, which isdefined by an outer diameter, wherein in the area of the backup lengththe outer diameter of the plastifying screw may be reduced, and/or thecylinder diameter of the barrel may be increased, and/or holes in screwlands may be provided.

According to another feature of the present invention, the extruder maybe a twin screw extruder with two plasticizing screws which rotate in asame direction. This configuration provides sufficient free spacebetween the screws for backup of melt so that the backup length caneasily expand backwards.

According to another feature of the present invention, the shutoff valvemay be an actively controllable shutoff valve.

According to another aspect of the present invention, a method ofoperating an injection molding machine of a type having an injectionplunger for pushing melt from an injection space anteriorly of aninjection plunger into an injection mold, with the method including thesteps of continuously operating a plasticizing screw of an extruder tocontinuously feed melt through a melt channel into the injection space,closing a passage through the melt channel to allow backup of meltbackwards into the extruder, and moving the injection plunger forward toinject a shot amount from the injection space into the injection mold.

Suitably, the melt channel is closed during the injection phase and/orafterpressure phase.

According to another feature of the present invention, the plasticizingscrew is continuously operated by a rotary drive at reduced speed duringthe injection phase and/or afterpressure phase.

BRIEF DESCRIPTION OF THE DRAWING

Other features and advantages of the present invention will be morereadily apparent upon reading the following description of currentlypreferred exemplified embodiments of the invention with reference to theaccompanying drawing, in which:

FIG. 1 is a schematic sectional side view of an injection deviceaccording to the present invention having a single injection plungershown in a retracted position shortly before injection of melt into aninjection mold; and

FIG. 2 is a schematic sectional side view of the injection device,illustrating the injection plunger in a forward position after injectionof melt into the injection mold.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Throughout all the Figures, same or corresponding elements are generallyindicated by same reference numerals.

Turning now to the drawing, and in particular to FIG. 1, there is showna schematic sectional side view of an injection device according to thepresent invention. This is one of two applications both filed on thesame day. Both applications deal with related inventions. They arecommonly owned and have the same inventive entity. Both applications areunique, but incorporate the other by reference. Accordingly, thefollowing U.S. patent application is hereby expressly incorporated byreference: “Injection Unit for Injection Molding Machines withContinuously Operating Plasticizing Unit”, application Ser. No.10/272,099. The injection device includes a plunger-type injection unit1, which includes a housing 4 and an injection plunger 2. The injectionplunger 2 has a conical pointed forward portion 2 a and reciprocates inthe housing 4. Movement of the injection plunger 2 within the housing 4is realized through actuation of a piston and cylinder unit, of whichonly piston rod 3 is shown for connection to the injection plunger 2 andwhich is operated by pressure medium. Formed in the housing 4 anteriorlyof the injection plunger 2 is an injection space 5 which receives meltfrom an extruder, generally designated by reference numeral 10, via amelt channel 8 terminating in a forward area of the injection space 5.The injection space 5 conically tapers in the direction of an injectionnozzle 6 through which melt is injected into an injection mold (notshown). Disposed upstream of the injection nozzle 6 is a shutoff valve 7which regulates a flow of melt from the injection space 5 into theinjection mold.

Plastic material is plasticized by the extruder 10 which has an extruderbarrel 12 and accommodates one or more plasticizing screws 13 (only oneplasticizing screw 13 is shown here by way of example) which can bedriven in a conventional manner by a motor and a transmission. Theextruder 10 may be a twin screw extruder in which the plasticizingscrews 13 rotate in a same direction. A feed hopper 14 is positioned ata rear portion of the extruder barrel 12 to supply plastic granulate tothe extruder 10. Arranged in the melt channel 8 is a further activelycontrollable shutoff valve 9 for regulating a flow of melt between ascrew anterior space 15 of the extruder 10 and the injection space 5.

In accordance with the present invention, the extruder 10 is configuredin such a manner that melt can backup in the extruder 10 to a backuplength R_(L1), when the shutoff valve 9 is closed, as shown in FIG. 2.Hereby, the plasticizing screw 13 is received within the barrel 12 in away that the crest 16 of the screw threads is spaced at a distance 17from the enclosing inside wall of the barrel 12. The spacing 17 betweenthe screw crest 16 and the barrel wall may be realized by reducing theouter diameter of the plasticizing screw 13, or by increasing thecylinder diameter of the barrel 12, or by arranging holes in screw landsin the area of the backup length R_(L1).

During operation of the injection molding machine, the extruder 10 iscontinuously operated and the injection space 5 is supplied with meltwhile the shutoff valve 9 is open. FIG. 1 shows a situation prior toinjection, in which the injection plunger 2 assumes a retractedposition, whereby reference character R_(L0) denotes the initial backuplength. When the injection space 5 is filled, the shutoff valve 9 in themelt channel 8 is closed, as illustrated in FIG. 2, while the shutoffvalve 7 in front of the injection nozzle 6 is opened. The injectionstroke of the injection plunger 2 can now be initiated by means of ahydraulic adjustment device (piston and cylinder unit) to fill theinjection mold of the injection molding machine with melt. FIG. 2 showsthe situation at the conclusion of the injection stroke. The injectionplunger 2 assumes its forwardmost position in the housing 4 and theinjection space 5 has its minimum volume.

Next, the shutoff valve 7 in front of the injection nozzle 6 is closed,while the shutoff valve 9 in the melt channel 8 is opened. As aconsequence, the injection plunger 2 is moved backwards by incomingmelt.

As the plasticizing screw 13 rotates uninterruptedly and material is fedvia the feed hopper 14, melt is continuously produced. The extruder 10may continue to run during the injection stroke and during the dwelltime of the injection-molded parts in the mold because during this time,when the shutoff valve 9 is closed, pressure, which builds up in thescrew anterior space 15, can propagate to the rear and a fill degree of1 can be generated little by little in the screw threads. The backuplength increases from a value R_(L0) to a value R_(L1). The continuousoperation of the extruder 10 is assured, especially when thinwalled formparts or short cycle times are involved, because there is not enoughtime for an excessive pressure buildup in the screw anterior space 15and melt can be buffered in this brief period in the area of the backuplength R_(L1), as shown in FIG. 2. A twin screw extruder with twoplasticizing screws 13, rotating in a same direction, has between theplasticizing screws sufficient free space (no closed screw threads),which enables a sufficient enlargement of the backup length from R_(L0)to R_(L1) and thus provides for a large enough buffer region. Hereby,the pressure, building up in the screw anterior space 15, drops normallylinearly from the screw anterior space 15 backwards to the free spacebetween the plasticizing screws.

Of course, this configuration of the afore-described twin screw extruderwith plasticizing screws rotating in same direction is described by wayof example only, and other configurations which generally follow theconcepts outlined here are considered to be covered by this disclosure.For example, the extruder 10 may be configured as a single screwextruder whereby in the area of the backup length the outer screwdiameter of the plasticizing screw 12 may be reduced and/or the cylinderdiameter of the barrel 12 may be increased, and/or arrangement of holes(not shown) in screw lands may be provided. Another variation involves aconstruction of the extruder 10 in the form of a twin screw extruderwith the two plasticizing screws arranged tangent to one another andthus do not mesh.

While the invention has been illustrated and described in connectionwith currently preferred embodiments shown and described in detail, itis not intended to be limited to the details shown since variousmodifications and structural changes may be made without departing inany way from the spirit of the present invention. The embodiments werechosen and described in order to best explain the principles of theinvention and practical application to thereby enable a person skilledin the art to best utilize the invention and various embodiments withvarious modifications as are suited to the particular use contemplated.

What is claimed as new and desired to be protected by Letters Patent isset forth in the appended claims and their equivalents:

1. A method of operating an injection molding machine of a type havingan injection plunger for pushing melt from an injection space anteriorlyof an injection plunger into an injection mold, said method comprisingthe steps of: continuously operating a plasticizing screw of an extruderto continuously feed melt through a melt channel into the injectionspace; closing a passage through the melt channel to allow backup ofmelt backwards into the extruder; and moving the injection plungerforward to inject a shot amount from the injection space into theinjection mold.
 2. The method of claim 1, and further comprising thestep of closing the melt channel during at least one of an injectionphase and afterpressure phase.
 3. The method of claim 1, and furthercomprising the step of operating a rotary drive to continuously operatethe plasticizing screw at reduced speed during at least one of aninjection phase and afterpressure phase.
 4. The method of claim 1,wherein the plasticizing screw defines an axis and is immobile in adirection of the axis.